Ohmmeter for measuring the internal resistance of a battery and directly reading the measured resistance value

ABSTRACT

An ohmmeter for measuring the internal resistance of a battery by an A.C. method. The output of a DC power circuit is converted into an alternating current of prescribed frequency through a DC - AC conversion circuit. The alternating current flows through a battery by way of a DC blocking device and the voltage drop due to the internal resistance of the battery is introduced into an AC voltmeter circuit. The internal resistance of the battery is indicated at the indicating portion of the AC voltmeter circuit so that it is read directly.

Field of Search ..324/29.5, 62, 64; Y 340/249; 136/182 United StatesPatent [1.91 [111 3,753,094 Furuishi et al. Aug. 14, 1973 OHMMETER FORMEASURING THE [56] References Cited INTERNAL RESISTANCE OF A BATTERYUNITED STATES PATENTS AND DIRECTLY READING THE MEASURED 2 811 446 mmWann 324/64 RESISTANCE VALUE 2,896,159 7/1959 Webster 324/64 [75]Inventors: Haruhba Flll'lllihi, Suita'; Yasunobu FOREIGN PATENTS "ORAPPIJCATIONS Iida, Osaka; Toshiaki Fukuoka, I Hirakata, a of Japan 7901,080 l/l954 Germany 324/62 [73] Assignee: Matsushita ElectricInduatrial Co. primary Examine,- Rben LCm-coran Osaka JapanAttorney-Stevens, Davis, Miller & Mosher [22] Filed: July 24, 1972 21Appl. No.: 274,809 [571 ABSTRACT An ohmmeter for measuring the internalresistance of 3 3"? a battery by an A.C. method. The output of a DC [6Cgmtanuatgon of Ser. No. 48,38l, June 22, 1970, power circuit isconverted into an alternating current an of prescribed frequency througha DC AC conversion r circuit. The alternating current flows through abattery [30] Foreign Appuufion by way of a DC blocking device and thevoltage drop July 1, 1969 Japan 44/53589 due m the internal resistanceof the battery is i duced into an AC voltmeter circuit. The internalresis- [52] Cl 324/29.5, 324/64, 340/249 m, f the battery is indicatedat thc indicating pop Int. Cl. ,G0ln 27/46 tion of the AC voltmetercircuit so tha it is rectly.

7 Claims, 10 Drawing Figures A 5 a Y 0 l\ Eu 5 BLOCK/N6 b 3 2 051005 0k9 Q QKD I F L1 9 y a BAT- m 1 e v p raw JM I A6 voumrm OIIMMETER FORMEASURING TIIE INTERNAL RESISTANCE OF A BATTERY AND DIRECTLY READING THEMEASURED RESISTANCE VALUE This is a continuation, of application Ser.No. 48,381, filed June 22, 1970 and now abandoned.

The present invention relates to an ohmmeter for a battery.

The ohmmeter for a battery in accordance with the present invention isadapted to measure the internal resistance of a battery on the sameprinciple as that of the standard for measurement brought forward by IEC(International Electrotechnical Commission).

As for the battery to be measured for internal resistance in the presentinvention, a storage battery can be measured as well as a dry batterysuch as a manganese dry cell, mercury battery and the like generally inuse.

The primary object of the present invention is to provide an ohmmeterfor a battery in which an alternating current flows through the batteryand the voltage drop due to the internal resistance of the battery isintroduced into an AC voltmeter circuit, thereby indicating themagnitude of the internal resistance of the battery so as to be directlyread.

Another object of the present invention is to provide an ohmmeter for abattery in which the circuit construction is simplified so that theoperation thereof may be conducted easily and it may be comfortablycarried.

Still another object of the present invention is to provide an ohmmeterfor a battery which is of use for checking and making uniform thequality of the batteries in the manufacturing process thereof.

Now the ohmmeter of the present invention will be described incomparison with the conventional type of ohmmeter of this kind inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of the circuit of the conventionalohmmeter,

FIG. 2 is a schematic diagram of the circuit of another conventionalohmmeter,

FIG. 3 is a schematic diagram of the circuit of still anotherconventional ohmmeter,

FIG. 4 is a diagram of the electrical circuit for explanation of theprinciple of the ohmmeter for a battery in accordance with the presentinvention,

' FIG. 5 is a block diagram of the ohmmeter is accordance with anembodiment of the present invention,

FIG. 6 is a view of the electrical circuit of the ohmmeter in accordancewith the embodiment of the present invention shown in FIG. 5,

FIG. 7 is a block diagram of the ohmmeter in accordance with anotherembodiment of the present invention in which the DC power circuit in theohmmeter shown in FIG. 6 is modified,

FIG. 8 is a block diagram of the ohmmeter in accordance with stillanother embodiment of the present invention in which the DCconstant-voltage circuit in the ohmmeter shown in FIG. 6 is modified,

FIG. 9 is a block diagram of the ohmmeter in accordance with stillanother embodiment of the present invention in which theconstant-current type DC AC conversion circuit in the ohmmeter shown inFIG. 6 is modified, and

FIG. 10 is a block diagram of the ohmmeter in accordance with a furtherembodiment of the present invention in which the AC voltmeter circuit inthe ohmmeter shown in FIG. 6 is modified.

There are two methods of measuring the internal resistance of a battery,that is, AC method and DC method. In both methods, there aredisadvantages in that the meter should be adjusted, the measured valueshould be calculated, or a precise measurement cannot be made, in theevent that the conventional bridge method or voltmeter method isapplied.

For instance, the Kohlrausch bridge method will be described as anexample of the measurement by the AC method. In this method, the batteryto be measured P is connected in one side of a bridge circuit as shownin FIG. I. The variable resistances R R are controlled to balance thecircuit so that the current may not flow through the detector D in thecircuit. The internal resistance R of the battery P is determined by thefollowing balance formula in which the value of the variable resistancesR and R is taken under balanced conditions. The condensers C, and C areblocking condensers for blocking the direct current from the battery P.

As an example of the measurement of the resistance of the battery by theDC method, Mances method will now be described. In this method, thebattery to be measured P is connected in one side of a Wheatstone bridgecircuit as shown in FIG. 2. The variable resistance R is controlled sothat the galvanometer G may not be effected by the switching on and offof the switch K. Under the balanced condition, the internal resistance Rof the battery P is determined by the following formula The voltmetermethod will now be described hereinbelow. The battery P to be measured,a voltmeter V and a known resistor R are connected in the circuit asshown in FIG. 3. First, the voltage across the battery is measured withthe switch K being opened. The measured voltage is indicated as Eo.Then, the voltage across the battery is measured with the switch K beingclosed. The measured voltage with the closed switch K is indicated asVo. In the case that the internal resistance of the voltmeter is muchlarger than the internal resistance of the battery R and the resistanceR, the internal resistance R of the battery is determined by thefollowing formula:

It should be however noticed that the measurement of the voltage V0across the battery with the closed switch K should be promptly conductedbefore the polarization of the battery commences.

As apparent from the above description regarding the prior art method ofmeasuring the internal resistance of the battery, adjustment orcalculation has been necessitated in the prior art.

The present invention overcomes the above defects of the prior art. Theprinciple thereof will now be described hereinbelow. As shown in FIG. 4,a resistor R, a DC blocking condenser C and the battery B to be measuredare connected in series across an AC power source P. The voltge drop bythe internal resistance R X of the battery B is measured with avoltmeter V coupled in parallel with the battery B. The internalresistance R is determined by the following formula and the internalresistance R may be indicated by the voltmeter using a scale ofresistance. Thus, the internal resistance R can be directly read on themeter.

Now the present invention will be described in detail with reference tosome embodiments thereof. In FIG.

5 the principal arrangement of the embodiments is shown with a blockdiagram, in which the reference character A indicates a DC power sourcecircuit, B indicates a DC constant voltage circuit, C indicates aconstant current type DC AC conversion circuit, D a DC blocking device,E .an AC volt-meter circuit, F a battery to be meauured, G, H currentterminals of the battery, and I and J indicate the voltage terminals ofthe battery to be measured.

In the above-described arrangement of the circuit,

' when an inconstant voltage provided by the'DC power source circuit Ais supplied to the DC constant voltage circuit B, the voltage iscontrolled to be constant and put into the constant current type DC ACconversion circuit C as an input power. By means of this constantcurrent type DC AC conversion circuit C, the direct current is convertedinto an alternating current (of sine wave form) having a prescribedfrequency, and the alternating current flows through the batteryconnected between the current terminals G and H by way of the DCblocking device D. At this time, the voltage across the battery Fmeasured between the voltage terminals I and J drops due to the internalresistance thereof. The voltage across the battery is put into the ACvoltmeter circuit E of high input inpedance as the detection input.Accordingly, if the indicator of the voltmeter circuit E is providedwith a resistance indicating scale, the indication of the pointerbecomes the indication in ohms of the internal resistance of thebattery.

Now an embodiment of the ohmmeter in accordance with the presentinvention will be described in detail referring to FIG. 6. In theembodiment, the DC power source circuit A includes batteries 1 so thatthe ohmmeter may be able to be used without a commercial AC powersource. The DC constant voltage circuit B consists of a constant voltagediode 2, a transistor 4, a resistor 3, and a capacitor 5 connected asshown in FIG. 6, and automatically controls the voltage across thebattery 1 by suppressing the fluctuation thereof to provide a stableconstant voltage power source.

The constant current type DC AC conversion circuit C consists of avoltage controlling circuit C, and a push-pull type DC AC conversioncircuit C, combined with the former. The voltage controlling circuit C,consists of a constant voltage diode 8, transistors 9 and 10, diodes 11and 13, a current transformer 14, resistors 6 and 7, and a capacitor 12connected as shown in FIG. 6. The push-pull type DC AC conversioncircuit C consists of transistors 15 and 16, an oscillation transformer18 and a resistor 17 connected as shown in the drawing. Through this DCAC conversion circuit C, a constant alternating current of a prescribedfrequency, e.g., a sine wave of mA and 1,000 cycles, is applied to thebattery F as a load. The AC voltmeter circuit E consists of an ACamplification circuit portion E and an indicating circuit portion E TheAC amplification circuit portion E, is a CR coupling amplificationcircuit composed of a collector grounded circuit of high input impedanceconsisting of a transistor 22, resistors 20, 21 and 23, and capacitors19 and 24 connected as shown in FIG. 6, and a high amplificationtransistor circuit consisting of a transistor 27 having an emittergrounded through the low AC im-.

pedance of capacitors 29 and 5 and resistors 25, 26, 28 and 30,connected as shown in the drawing. The constant DC voltage used for thiscircuit E, is obtained at the output terminals of the DC constantvoltage circit B. When the constant alternating current flows throughthe battery F between the current terminals Gand H, the voltage drop dueto the internal resistance of the battery can be detected at the voltageterminals I and J. The indicating circuit portion E, of the AC voltmetercircuit E consists of a semi-conductor rectifier 32 and a DC meter 33connected as shown in FIG. 6, and rectifies the output current amplifiedby the AC amplification circuit E, so that the pointer in the DC meter33 provided with a resistance scale indicates the internal resistance ofthe battery F. The DC blocking device D consisting of a non-polarizationcapacitor is connected in series between theconstant current type DC ACFIG. 6 is illustrated. In this modification, a DC power circuit Aconsisting of a commercial AC power source 34, a transformer 35 and asemiconductor rectifier36 is substituted for the DC power source circuitA so that the ohmmeter may be used for a long time with a commercial ACpower source.

. In FIG. 8 a modification of the ohmmeter shown in FIG. 6 isillustrated with a block diagram, in which a DC constant voltage circuitB consisting of a constant voltage diode 37, transistors '39 and 42,resistors 38, 40 and 41, and'a capacitor 43 connected as shown in thedrawing is substituted for the DC constant voltage circuit B.

In the DC constant voltage circuit B in the ohmmeter shown in FIG. 6,when the voltage across the emitter and base of the transistor 4 variesdue to temperature variation, the output voltage is varied, whichresults in error in indication. In order to prevent the error inindication due to the temperature variation, another transistor 39 of adifferent type is connected with the transistor 42 so that the variationin voltage due to the temperature variation across the emitter and baseof the respective transistors 39 and 42 may cancel each other. Thus, theoutput voltage is stabilized for the effect of the variation intemperature. The above stabilization can be simply explained by thefollowing formulae (1) and (2). The relation between the output voltageE of the DC constant voltage circuit B,.the terminal voltage E of theconstant voltage diode 2,-and the voltage E across the emitter and baseof the transistor 4 is EL=E E I and the relation betweenthe outputE ofthe DC constant voltage circuit B, the terminalvoltage E, of

the constant voltage diode 37, the voltage E across the emitter and baseof the transistor 42, and the voltage E across the emitter and base ofthe transistor 39 is Accordingly, if the transistors 39 and 42 areselected so that the relation E z E is satisfied, the formula (2)becomes E E which makes the output voltage be independent of the voltageacross the emitter and base of the transistors.

Inthe circuit connetion of the DC constant voltage circuit B, thetransistor 42 is connected in series between the DC power source A andthe constant current type DC AC conversion circuit C. The collector ofthe transistor 42 is connected with the negative side of the DC powersource A, the emitter thereof is connected with the constant currenttype DC AC conversion circuit C, and the base thereof is connected withthe e'mitterof the transistor 39 and connected through a reslstor 4 1with the negative side of the DC power source A. The baseof thetransistor 39 is connected with the positive side of the DC power sourceA through a constant voltage diode 37 and connected with the negativeside of the DC power source A through a resistor 38. The collector ofthe transistor 39 is connected with the positive side of the DC powersource A through a resistor 40.

. In FIG. 9, another embodiment of the ohmmeter of the present inventionis shown in which the ohmmeter shown in FIG. 6 is modified. The constantcurrent type DC AC conversion circuit C in the ohmmeter shown in FIG. 6is substituted for by a modified constant current type DC- AC conversioncircuit C. The constant current type DC AC conversion circuit C consistsof a voltage control circuit C, and a push-pull type DC- AC conversioncircuit C The voltage control circuit C, consists of a control circuitconsisting of a detecting transistor 44, acontrol transistor 46 andresistors 45 and 47, a voltage feedback circuit consisting of arectifier 48 for rectifying the feedback voltage across the third coil80 of the oscillation transformer 56 to put signals into the base of thecontrol transistor 46, a resistor 57, a capacitor 49 and the third coil80, and a current feedback circuit consisting of a rectifier 52 forrectifying the feedback current through the second coil 81 of theoscillation transformer 56 to put the signals into the base of thedetection transistor 46, a resistor 50 and a capacitor 51. The push-pulltype DC 4 AC conversion circuit C, consists of transistors 53 and 54, acollector coil 82, a base coil 83, the foregoing second and third coils81 and 80, an oscillation transformer 56 and a resistor 55 connected asshown in FIG. 9. While in the constant current type DC AC conversioncircuit C in the ohmmeter shown in FIG. 6 the input into the DC ACconversion circuit C, is controlled only by the current feedback to makethe load current constant, in the constant current type DC AC conversioncircuit C in the ohmmeter shown in FIG. 9 the input into the DC ACconversion circuit C, is controlled by both the voltage and currentfeedbacks to make the load current constant. Therefore, in the ohmmeterin accordance with the embodiment shown in FIG. 9, the efficiency ofcontrol is much improved and the synchronization can be easily conductedbetween the oscillation and the load current. Accordingly, a precisemeasurement can be accomplished in the case that a synchronous rectitiercircuit is substituted for the rectifier 32 for converting the AC outputfrom the AC amplification circuit portion E, into the direct current inthe AC voltmeter E.

The constant current operation of the circuit C will now be describedhereinbelow.

In the eventthat the load current isreduced, the volt age drop acrossthe resistor 50 is reduced and the base potential of the detectiontransistor 44 is lowered. Accordingly, the collector current isreducedand the voltage drop across the resistor 45 is reduced, and the voltageacross the base and emitter of the control transistor 46 is raised,which results in reduction in the voltage across the emitter andcollector of the control transis- On the other hand, if the load currentis reduced, the

output voltage is raised due to the lightload at the oscillationtransformer 56. Then the voltage across the resistor 57 is raised andthe DC voltage rectified through the rectifier 48 is also raisedresulting in the rise of the voltage across the base and emitter of thecontrol transistor 46. On account of the above, the voltage across thecollector and emitter is lowered 'and the input voltage of the DC ACconversion circuit C, is raised. In response to the rising of the inputvoltage of the DC AC conversion circuit C the output voltage is raisedand the output current (load current) is increased to return to itsinitial condition. In'the case that the load current is increased,entirely the reverse operation .is conducted.

In FIG. 10, another embodiment of the present invention is shown inwhich the AC amplification circuit E, in the AC voltmeter circuit'E ofthe ohmmeter embodiment shown in FIG. 6 is substituted for by an ACamplification circuit E, consisting of transistors 61 and 66, resistors59,60, 62, 64, 65,67, 69 and 71, capacitors 58, 63, 68 and 70, and aresistance element 72 having a negative temperature coefficient. The ACamplification circuit E, corresponds to a circuit consisting of the ACamplification circuit E, and a negative feedback circuit E, connectedbetween the emitter of the transistor 22 and the collector of thetransistor 27 in the circuit E, (FIG. 6). In FIG. 10, the negativefeedback circuit E, is connected between the emitter of the transistor61 and the collector of the transistor 66. By the provision of thenegative feedback circuit E, the temperature characteristic is improved.Since the coefficient of the temperature characteristic of the ACamplification circuit E, shown in FIG. 6 is positive, the negativefeedback circuit E having a negative temperature coefficient is disposedbetween the emitter of the transistor 61 and the collector of thetransistor 66 in the AC amplification circuit E, as shown in FIG. 10 inorder to compensate the positive temperature variation. The negativefeedback circuit E is composed of a thermistor 72 and a resistor 71connected in series for providing a proper negative temperaturecoefficient.

In accordance with the present invention as described above, it ispossible to read directly the internal resistance of a battery through awide range and with high accuracy. Furthermore, the ohmmeter can be madeinto a compact portable size and it can be operated with a DC powersource as well as an AC power source. The ohmmeter in accordance withthe present invention is useful, as apparent from the above, for qualitycontrol in the battery manufacturing process.

What is claimed is:

1. An ohmmeter for measuring the internal resistance of a batterycomprising a DC power source circuit;

a DC constant voltage circuit for producing a DC constant voltage outputfrom the DC power of said DC power source circuit,

a DC-AC conversion circuit for converting said DC constant voltageoutput to an AC constant current having a relatively high frequency;said DC constant voltage circuit being a series-type constant voltagecircuit comprising a control transistor having a collector connected toone output terminal of said DC power source circuit, an emitterconnected to one input terminal of said DC-AC conversion circuit, and abase connected to said one output terminal of said DC power sourcecircuit through a resistor; and a reference transistor having an emitterconnected to the base of said control transistor and a base connected tothe other output terminal of said DC power source circuit through aconstant voltage diode and also to said one output terminal of said DCpower source circuit through another resistor;

a DC blocking device, said AC constant current being applied to abattery to be measured through said DC blocking device, said DC blockingdevice preventing DC current from flowing from the battery; and i an ACvoltmeter circuit including an indicator connected acrossthe battery formeasuring the AC voltage drop of the battery due to its internalresistance, said indicator of said AC voltmeter circuit being adapted toindicate directly the internal resistance of the battery.

2. An ohmmeter for measuring the internal impedance of a batteryaccording to claim 1 wherein said control transistor and referencetransistor have the same temperature characteristic with respect to thevoltage across the emitter and base thereof, and one of said transistorsis of PNP type and the other of said transistor is of NPN type.

3. An ohmmeter for measuring the internal resistance of a batterycomprising a DC power source circuit;

a DC constant voltage circuit for producing a DC constant voltage outputfrom the DC power of said DC power source circuit;

a DC-AC conversion circuit for converting said DC constant voltageoutput to an AC output of a constant current having a sine wave of apredetermined frequency, said DC-AC conversion circuit including meansfor maintaining the current and voltage of said AC output at constant;and further comprising means for converting the DC voltage to an ACvoltage, said converting means including an oscillation transformerhaving a secondary coil for producing said AC constant current and acontrol circuit connected in circuit between said DC constant voltagecircuit and said converting means for controlling said DC constantvoltage produced by said DC constant voltage circuit in accordance witha feed-back current and a feed-back voltage supplied 6 from saidconverting means thereby applying the controlled DC constant voltage tosaid converting means; current feed-back means including a first circuitconnected between said secondary coil and the battery to be measured fortaking out a current component relating to said AC constant currentapplied to said battery, and a rectifying circuit connected between saidfirst circuit and said control circuit for producing said feed-backcurrent by rectifying said current component; and voltage feedback meansincluding a third coil of said oscillation transformer for taking out avoltage component proportional to a voltage induced to said secondarycoil, and a rectifying circuit connected between said third coil andsaid control circuit for producing said feed-back voltage by rectifyingsaid voltage component;

a DC blocking device, said AC constant current being applied to abattery to be measured through said DC blocking device, said DC blockingdevice preventing DC current from flowing from the battery; and

an AC voltmeter circuit including an indicator connected across thebattery for measuring the AC voltage drop of the battery due to itsinternal resistance, said indicator of said AC voltmeter circuit beingadapted to indicate directly the internal resistance of the battery.

4. An ohmmeter for measuring the internal resistance of a batterycomprising: A DC power source circuit;

a DC constant voltage circuit including a first control transistorconnected in series between said DC power source and a voltage controlcircuit for producing a DC constant voltage from the DC power of said DCpower source circuit;

a DC-AC conversion circuit comprising said DC voltage control circuitand an oscillation circuit, said DC voltage control circuit including asecond control transistor connected in series between said DC constantvoltage circuit and said oscillation circuit for controlling said DCconstant voltage thereby supplying said oscillation circuit with a DCcontrolled voltage, and said oscillation circuit including a pair ofpush-pull connected output transistors for converting said DC controlledvoltage to an AC controlled voltage and a transformer having aprimary'winding connected to said pair of output transistors and asecondary winding for producing an AC output voltage proportional tosaid AC controlled voltage;

connecting means for connecting said secondary winding to a battery tobe measured, said means including a DC blocking device for preventing DCcurrent from flowing from said connected battery and terminal meansincluding current and voltage terminals for supplying said connectedbattery with a current from said output voltage of said secondarywinding through said current terminals and takingout the voltage drop ofsaid connected battery through said voltage terminals;

means for detecting variation of said current supplied to said batterythrough said connecting means and applying a signal relating to saidvariation to said second control transistor for controlling the same;and

indicator means comprising an AC amplifier connected to said voltageterminals for amplifying said voltage drop of said battery and anindicator circuit including a rectifier and an indicator connected tosaid amplifier through said rectifier for indicating directly theinternal resistance of said battery by measuring said amplified voltagedrop after rectification.

5. An ohmmeter according to claim 4, wherein said DC power sourcecircuit includes a battery as a DC power source and said DC blockingdevice includes a non-polarized capacitor.

6. An ohmmeter according to claim 4, wherein said AC amplifier of saidindicator means includes an input transistor having a collectorconnected to ground.

thereby providing a large input impedance, an output take out thevoltage drop of said connected battery. i i

1. An ohmmeter for measuring the internal resistance of a batterycomprising a DC power source circuit; a DC constant voltage circuit forproducing a DC constant voltage output from the DC power of said DCpower source circuit, a DC-AC conversion circuit for converting said DCconstant voltage output to an AC constant current having a relativelyhigh frequency; said DC constant voltage circuit being a series-typeconstant voltage circuit comprising a control transistor having acollector connected to one output terminal of said DC power sourcecircuit, an emitter connected to one input terminal of said DC-ACconversion circuit, and a base connected to said one output terminal ofsaid DC power source circuit through a resistor; and a referencetransistor having an emitter connected to the base of said controltransistor and a base connected to the other output terminal of said DCpower source circuit through a constant voltage diode and also to saidone output terminal of said DC power source circuit through anotherresistor; a DC blocking device, said AC constant current being appliedto a battery to be measured through said DC blocking device, said DCblocking device preventing DC current from flowing from the battery; andan AC voltmeter circuit including an indicatOr connected across thebattery for measuring the AC voltage drop of the battery due to itsinternal resistance, said indicator of said AC voltmeter circuit beingadapted to indicate directly the internal resistance of the battery. 2.An ohmmeter for measuring the internal impedance of a battery accordingto claim 1 wherein said control transistor and reference transistor havethe same temperature characteristic with respect to the voltage acrossthe emitter and base thereof, and one of said transistors is of PNP typeand the other of said transistor is of NPN type.
 3. An ohmmeter formeasuring the internal resistance of a battery comprising a DC powersource circuit; a DC constant voltage circuit for producing a DCconstant voltage output from the DC power of said DC power sourcecircuit; a DC-AC conversion circuit for converting said DC constantvoltage output to an AC output of a constant current having a sine waveof a predetermined frequency, said DC-AC conversion circuit includingmeans for maintaining the current and voltage of said AC output atconstant; and further comprising means for converting the DC voltage toan AC voltage, said converting means including an oscillationtransformer having a secondary coil for producing said AC constantcurrent and a control circuit connected in circuit between said DCconstant voltage circuit and said converting means for controlling saidDC constant voltage produced by said DC constant voltage circuit inaccordance with a feed-back current and a feed-back voltage suppliedfrom said converting means thereby applying the controlled DC constantvoltage to said converting means; current feed-back means including afirst circuit connected between said secondary coil and the battery tobe measured for taking out a current component relating to said ACconstant current applied to said battery, and a rectifying circuitconnected between said first circuit and said control circuit forproducing said feed-back current by rectifying said current component;and voltage feed-back means including a third coil of said oscillationtransformer for taking out a voltage component proportional to a voltageinduced to said secondary coil, and a rectifying circuit connectedbetween said third coil and said control circuit for producing saidfeed-back voltage by rectifying said voltage component; a DC blockingdevice, said AC constant current being applied to a battery to bemeasured through said DC blocking device, said DC blocking devicepreventing DC current from flowing from the battery; and an AC voltmetercircuit including an indicator connected across the battery formeasuring the AC voltage drop of the battery due to its internalresistance, said indicator of said AC voltmeter circuit being adapted toindicate directly the internal resistance of the battery.
 4. An ohmmeterfor measuring the internal resistance of a battery comprising: A DCpower source circuit; a DC constant voltage circuit including a firstcontrol transistor connected in series between said DC power source anda voltage control circuit for producing a DC constant voltage from theDC power of said DC power source circuit; a DC-AC conversion circuitcomprising said DC voltage control circuit and an oscillation circuit,said DC voltage control circuit including a second control transistorconnected in series between said DC constant voltage circuit and saidoscillation circuit for controlling said DC constant voltage therebysupplying said oscillation circuit with a DC controlled voltage, andsaid oscillation circuit including a pair of push-pull connected outputtransistors for converting said DC controlled voltage to an ACcontrolled voltage and a transformer having a primary winding connectedto said pair of output transistors and a secondary winding for producingan AC output voltage proportional to said AC controlled voltage;connecTing means for connecting said secondary winding to a battery tobe measured, said means including a DC blocking device for preventing DCcurrent from flowing from said connected battery and terminal meansincluding current and voltage terminals for supplying said connectedbattery with a current from said output voltage of said secondarywinding through said current terminals and takingout the voltage drop ofsaid connected battery through said voltage terminals; means fordetecting variation of said current supplied to said battery throughsaid connecting means and applying a signal relating to said variationto said second control transistor for controlling the same; andindicator means comprising an AC amplifier connected to said voltageterminals for amplifying said voltage drop of said battery and anindicator circuit including a rectifier and an indicator connected tosaid amplifier through said rectifier for indicating directly theinternal resistance of said battery by measuring said amplified voltagedrop after rectification.
 5. An ohmmeter according to claim 4, whereinsaid DC power source circuit includes a battery as a DC power source andsaid DC blocking device includes a non-polarized capacitor.
 6. Anohmmeter according to claim 4, wherein said AC amplifier of saidindicator means includes an input transistor having a collectorconnected to ground thereby providing a large input impedance, an outputtransistor having an emitter connected to ground through a low ACimpedance thereby providing a high amplification factor and negativefeedback means including a resistance element having a negativetemperature coefficient for connecting the collector of said outputtransistor to the emitter of said input transistor.
 7. An ohmmeteraccordingto claim 4, wherein said terminal means includes a pair of saidcurrent terminals adapted to be connected across said battery to bemeasured and a pair of said voltage terminals adapted to take out thevoltage drop of said connected battery.